I can see why it's unsettling, OP. With the assault as well (which I assume was upsetting and frightening but didn't result in serious injury?) it points to this not being a one off, unfortunate incident, but a couple who are not bothered about societal norms.
If it makes you feel any safer, there's no telling what the guy who was arrested has done other than not attend court. It could be for something that, as a neighbour, you are at risk regarding, but it could be for drink driving or TV license dodging where you are probably at little to no risk. I think the assault is more worrying.
I would make sure anything that they do to you is documented so report any future assaults or any harassment. Ensure you have good locks on your flat door and secure windows. Some sort of camera that covers your entry would be good if you can get away with it, I know it can be difficult in shared properties. If you can't have anything outside the door then something inside may be better than nothing, but may not be worth it. Are there others in your building or is it just the two flats? If others, finding out if the couple treat other people the way they've treated you might be useful and give you more of an idea of risk.
I would absolutely ignore people who say it's none of your business. This is your life. Don't build it up into something it's not, there is risk everywhere, and you don't know much yet, it may be nothing. You still need to have a life worth living not tie yourself up in knots of worry and avoid things you love. But don't ignore it. At the moment you don't have much to go on so you mainly need to stay alert. Knowing what's going on around you is sensible. The police are not your first line of defence - you are.